Industry has long been concerned with the clean-up, by the removal of dissolved organic pollutants which are toxic in nature, of process water streams and wastewater streams so as to render them suitable for reuse or for discharge either to a municipal treatment plant or to the environment. The concern has grown greater in recent years as the standards for wastewater discharge have become more stringent, particularly in the United States. As a result, there have been significant attempts made to recycle process water streams. Various treatment techniques have been developed over the years attempting to purge such process waters of organic pollutants.
U.S. Pat. No. 4,518,502 to Burns et al., issued May 21, 1985, shows a method for countercurrent extraction of a process water stream using a solvent such as n-pentane which is particularly suitable for the extraction of chlorinated hydrocarbon pesticides, including DDT, chlordane and others. The solvent which dissolves in the process water stream is removed by gas sparging, and the solvent stream is subsequently separated from the pesticides by gas sparging to vaporize the solvent. Other patents, such as U.S. Pat. No. 4,162,902, use steam-stripping to vaporize the solvent from the extract, and steam-stripping has also been taught to remove solvent from the raffinate following such an extraction, as shown in U.S. Pat. No. 3,449,244.
In oil refineries, contaminants which are valuable in catalytic cracking operations are sometimes extracted from sour refinery water as by the use of gas oil, and the phenol-enriched gas oil is then used as a feed for a fluid catalytic cracking unit, as disclosed in U.S. Pat. No. 4,746,434, issued May 24, 1988. Other similar disclosures of such refinery operations exist.
Although solvent extraction has been proposed for removal of organic pollutants from aqueous streams for a number of years, in very many instances the costs of solvent have been too high to be cost-competitive with other competing systems. In many instances, pure solvents are preferred to facilitate solvent recycling; however, because of the relatively high cost of procuring and operating with pure solvents, including providing make-up solvent, such processes have had economic disadvantages. In some instances, the cost of such pure solvents may be sufficiently high that, even when gas sparging is used, the costs are sometimes still not competitive. Heretofore, adsorption of these organic pollutants on activated carbon has been one viable alternative. However, in the United States, the Environmental Protection Agency's more recent regulations render it no longer feasible to dispose of organic pollutant-contaminated spent activated carbon by landfill, thus requiring either the regeneration of the activated carbon extractants, a relatively costly proposition, or the incineration of them, adding significantly to the expense.
Because of the growing interest worldwide in combating pollution and groundwater contamination, this subject remains of important interest and improved methods of treatment which are efficient and which are particularly capable of economical practice continue to be sought. As a result, the search continues for more economical and cost-efficient ways of removing water-soluble organic pollutants from process or wastewater streams, and in this respect, the minimization of both the cost of operating materials and the cost of the extraction plant installation itself is a substantial concern.